MCAT Kaplan Biology Chapter 12

hemizygous

only one allele is present for a given gene

complete dominant:

one dom allele and one recessive allele, the dominant will completely mask

penetrance

-proportion of individuals in a population that actually expresses the phenotype

expressivity

defined as varying phenotypes despite identical genotypes

-if expressivity is constant, then all organisms with same phenotype would have same genotype

-but, if expressivity is variable, individuals with same genotype could have varying phenoty[es

mendel's 1st law: law of segregation

-4 tenets of 1st law:

1. genes exist in alternative forms in the form of alleles

2. an organism has 2 alleles for each gene, one from each parent

3. 2 alleles segregate during meiosis, so gametes carry only 1 allele

4. if 2 alleles of an organism are different, one will be expressed (dominant), the other will be silent

wild type

-basically alleles that are considered "normal"/"natural", aka ubiquitous

-the less common allele is called "mutant"

nucleotide level mutations

Point mutations

Frameshift mutations:

chromosomal mutations

Large Deletion mutations

• Duplication mutations

• Inversion mutations

• Insertion mutations

• Translocation mutations

genetic drift

-defined as changes to composition of gene pool due to change

-more pronounced in small populations

monohybrid cross

-a cross when only one strait is studied is called monohybrid

-parents are the P generation

-filial (children) are the F generation

Grandparents are P, parents are F1, you are F2

test cross

-used to determine unknown phenotype

-basically, you cross an unknown genotype with a homozygous recessive

dihybrid cross

cross of 2 different genes

sex-linked crosses

gene mapping

-we can analyze recombination frequency to get a map representing relative distance between genes on a chromosome

-tighly linked genes have recombination frequencies close to 0, while weakly linked genes approach 50%

-one map unit or centimorgan is a 1 percent change of recombination, so 25 centimorgans apart means 25% of gametes show recombination between two genes

hardy weinberg equilibrium

when a population is:

1. very large (no genetic drift)

2. no mutations that affect gene pool

3. mating between individuals is random

4. no migration of indiviuduals

5. genes are all equally successful at reproducing

than the allele frequencies will remain constant.

modern synthesis model tenets

sometimes called neo-Darwinism, accounts for mutation and recombination as mechanisms of variation and considers differential reproduction to be the mechanism for reproductive success.

modes of natural selection

speciation

-defined as largest group of organisms capable of breeding to form fertile offspring

-formation of new species is called speciation

-after enough time where species are geographically separated, changes lead to isolation and species can no longer interbeed

reproductive isolation mechanisms

prezygotic mechanisms

postzygotic mechanisms

patterns of evolution

divergent evolution

convergent evolution

parallel evolution

measuring evolutionary time

degree of genomic similarity with amount of time species split off

-known as the molecular clock model

codominance

if there are 2 dom alleles at same time, both are expressed, like AB blood

Incomplete dominance

heterozygote expresses phenotype intermediate between homozygous genotypes. basically neither allele is dominant, mix of both

Hemizygous example

male sex chromosome

Incomplete dominance example

red flower + white flower = pink flower

Complete dominance example

A pea plant with all purple flowers and a pea plant with all white flowers produce a pea plant with all purple flowers

Penetrance example

Huntington's disease, those with the huntingtin gene that have 40 sequence repeats have full penetrance (100% show symptoms of huntington's disease), those with fewer sequence show high penetrance, and those with even fewer have low penetrance or nonpenetrance

expressivity example

A person may have a gene for familial hypercholesterolemia but can have a mild or severe form of the disease.

Mendel's Second Law of independent assortment

-the inheritance of one gene does not affect the inheritance of another

-exception are linked genes

-independent assortment increases genetic diversity of gametes

What phase of meiosis does Mendel's 1st law occur in?

During anaphase

What phase of meiosis does Mendel's 2nd law occur in?

Prophase I

Point Mutations

occur when one nucleotide in DNA (A, C, T, or G) is swapped for another. These can be subcategorized as silent, missense, or nonsense mutations.

Silent mutations

occur when the change in nucleotide has no effect on the final

protein synthesized from the gene. This most commonly occurs when the changed

nucleotide is transcribed to be the third nucleotide in a codon because there is

degeneracy (wobble) in the genetic code.

Missense mutations

occur when the change in nucleotide results in substituting

one amino acid for another in the final protein.

Nonsense mutations

occur when the change in nucleotide results in substituting a

stop codon for an amino acid in the final protein

Frameshift Mutations

moving of the 3-letter transcriptional reading frame.

2 Types

Insertions and deletions

Insertions

occur when a segment of DNA is moved from one

chromosome to another. Small insertion mutations (including those where the inserted DNA is not from another chromosome) are considered frameshift

mutations, as described previously.

Deletions

occur when a large segment of DNA is lost from a

chromosome. Small deletion mutations are considered frameshift mutations, as described previously.

Genetic leakage

a flow of genes between species through hybrid offspring.

Why would genetic leakage increase within the last century?

In the 20th century, advances were made in the understanding and technology to utilize genetics, thus scientists were able to create fertile hybrids.

founder effect

small group gets reproductively isolated, causing inbreeding and amplifies characters of smaller population

What causes inbreeding depression?

The species becomes too large for the resource base. Populations have low genetic diversity. Genetically similar parents mate and produce superior offspring. The number and variety of species increase.

Translocation mutations

occur when a segment of DNA from one chromosome is

swapped with a segment of DNA from another chromosome

Inversion mutations

occur when a segment of DNA is reversed within the

chromosome.

Duplication mutations

occur when a segment of DNA is copied multiple times in

the genome.

Recombination frequency

likelihood of 2 alleles being separated during crossing over in meiosis.

Chiasma

genes very close together are less likely to be separated

Hardy-Weinberg Equation

p+q=1

or

p^2 + 2pq + q^2 = 1

What does p^2 represent in the Hardy-Weinburg Principle?

dominant genotypes and phenotypes in a population

What does 2pq represent in the Hardy-Weinburg Principle

heterozygous genotypes and phenotypes in population

What does q^2 represent in the Hardy-Weinburg Principle?

recessive genotypes and phenotypes in a population

What does the p+q=1 version of the Hardy-Weinberg equation represent?

tells you about the frequency of alleles in the population

What does the p^2+2pq + q^2 version of the Hardy-Weinberg equation represent?

tells us about the frequency of genotypes and phenotypes in the population.

punctuated equilibrium

changes in some species occur rapidly in bursts due to changing environment, instead of evenly over time

Inclusive Fitness

refers to the idea that an organism's success is based on the number of offspring, success in supporting offspring, and the ability of the offspring to then support others; survival of offspring or relatives ensures appearance of genes in subsequent generations.

Natural Selection

states that chance variations exist between individuals and that advantageous variations afford the most opportunities for reproductive success/survival.

Differential Reproduction

when mutation results in favorable organism reproductive success, that will more likely pass to next generation

stabilizing selection

keeps phenotypes within specific range by selecting against extremes

-i.e. fetus weight. fetuses too light are unhealthy, fetuses too heavy can cause trauma during delivery

directional selection

adaptive pressures, causing emergence of initially extreme phenotype

disruptive selection

-two extreme phenotypes are selected over the norm

-i.e. finches in the Galapagos island, either had small or large beaks, no in ebtween

What makes disruptive selection possible?

polymorphisms, or naturally occurring differences between members of the same population

What is an example of divergent evolution?

adaptive radiation: idea that there is rapid rise of many species from a common ancestor

divergent evolution

when two or more species sharing a common ancestor become more different over time

Parallel evolution

occurs when 2 species sharing a common ancestor evolve in similar ways due to analogus selection.

Convergent evolution

occurs when 2 species not sharing a recent ancestor evolve to become more similar due to analogus selection pressures.

Species

largest group of organisms capable of breeding to form fertile offspring

prezygotic mechanisms

prevent formation of zygote completely

-includes:

1. temporal isolation

2. ecological isolation

3. behavioral isolation

4. gametic isolation

temporal isolation

breed at different times

ecological isolation

live in different niches within same territory

behavioral isolation

lack of attraction

gametic isolation

intercourse can occur, but no fertilization

postzygotic mechanisms

gamete fusion but yield nonviable or sterile offspring

-includes:

1. hybrid inviability

2. hybrid sterility

3. hybrid breakdown

hybrid inviability

forming zygote that cannot develop to term

hybrid sterility

offspring cannot reproduce

hybrid breakdown

first generation offspring can reproduce, second generation cannot

Why are sex-linked disorders more common in males than in females?

males only have 1 X chromosome, and are hemizygous for genes on X chromosome, thus a recessive allele is sufficient for a recessive phenotype

transposons

can insert and remove themselves from genome